chore: Upgrade to Java 11

[aababilov]

Google has finally upgraded to Java 11, so the open-source validator
does not have to use the old Java 8 (see pull #595 and pull #759).

Java memory is reduced from 12 to 8 GiB to make the tests pass.

[aababilov]

End-to-end 100 test failed on Ruler, Norway

9s
0s
Run java -Xmx12G -Xms12G -jar main/build/libs/*.jar --url http://transitfeeds.com/p/ruter/240/latest/download --output_base output --country_code no --storage_directory ruter.zip
OpenJDK 64-Bit Server VM warning: INFO: os::commit_memory(0x0000000500000000, 12884901888, 0) failed; error='Not enough space' (errno=12)
#
# There is insufficient memory for the Java Runtime Environment to continue.
# Native memory allocation (mmap) failed to map 12884901888 bytes for committing reserved memory.
# An error report file with more information is saved as:
# /home/runner/work/gtfs-validator/gtfs-validator/hs_err_pid1746.log

[aababilov]

Reducing memory from 12 to 8 GiB made a miracle - now the tests pass!

[botanize]

Should this PR also update the java version requirement in README.md?

[aababilov]

Should this PR also update the java version requirement in README.md?

Done.

[barbeau]

LGTM

[aababilov]

Thank you!